The invention in particular relates to the ball-and-socket joints used in the control mechanism of a helicopter.
A helicopter comprises a primary rotor and a tail rotor on which blades are mounted in an articulated manner. By changing the incidence of the blades, the pilot can steer the helicopter in the desired direction.
More specifically, the pilot actuates a control stick that acts on a mechanism for controlling the incidence of the blades. This mechanism comprises an oscillating plate that is mounted on the rotor and that is connected to each of the blades of the rotor by connecting rods and levers. This oscillating plate can pivot around two directions perpendicular to the main axis of the rotor so as to drive the rotation of the levers. A lever is connected to a connecting rod by a ball-and-socket joint, which makes it possible to transmit the rotational movement and modify the incidence of the blades.
A ball-and-socket joint comprises a first ring, for example cylindrical, and a second spherical ring. The first ring comprises a spherical housing in which the second ring is arranged. The first ring is for example secured to a lever, while the second ring is secured to a connecting rod. A lining, or trim, is inserted between the outer ring and the inner ring. It is a solid lubricant with a very low frictional torque. This trim makes it possible to keep the rings of the ball-and-socket joint centered on a same point and limits the friction during the rotation of one ring relative to the other.
However, after a certain number of flight hours, the ball-and-socket joints become worn and a clearance is created between the outer ring and the inner ring. Thus, a relative movement between the outer ring and the inner ring is possible, with the result that the ball-and-socket joint no longer has the desired kinematics. In practice, that clearance within the ball-and-socket joints can make the helicopter uncontrollable. The wear of the ball-and-socket joints depends on the load conditions of the helicopter and on the flight conditions. Thus, the ball-and-socket joints of the oscillating plate do not each have the same wear speed.
It is therefore important to measure the degree of wear of each ball-and-socket joint of the control mechanism for the helicopter regularly. Currently, this measurement is done mechanically, i.e., one ring among the outer ring and the inner ring of the ball-and-socket joint is blocked while the other ring is subjected to a load. By measuring the relative movement between the outer ring and the inner ring, it is possible to deduce the relative clearance thereof between the rings of the ball-and-socket joint, and therefore the wear of the latter. The problem with this method is that it requires the complete disassembly of the oscillating plate.
Furthermore, in light of the number of ball-and-socket joints included in an oscillating plate, the mechanical measurement of the wear of the set of ball-and-socket joints is particularly long, with the result that the helicopter can be immobilized for several hours, or even several days. To resolve these drawbacks, it remains known from US-A-2009/0087253 to incorporate a capacity sensor comprising two electrodes, respectively connected to the two rings of the ball-and-socket joint, into the ball-and-socket joint. The two rings being separated by an electrically insulating lining, they therefore form a capacitor structure with the latter. The variation of the capacity of the capacitor thus formed reflects the wear of the ball-and-socket joint. However, it is difficult to obtain a precise capacity value of the capacitor via a direct measurement.